special defeat conditions

Is it possible to set more than 12 months of game time as "special defeat conditions" in the Heroes of Might and Magic 3: Shades of Darkness map editor?

I doubt there's a way to do that in the Heroes of Might and Magic 3: Shades of Darkness editor, at least not in the way it was originally intended. Unless you come up with some "workarounds." For example, you could set victory conditions for a "computer player" in an isolated dungeon – to retrieve an artifact located behind the walls of a fortress, guarded by tons of archangels, and the "computer" could only manage to defeat them (and retrieve the artifact) after 12 months, accumulating troops while the main game events unfold outside the isolated dungeon. Or, as an alternative, you could try to modify the editor and "tweak" the ability to set "13 (or more) months" within it.